为什么这个油漆组件不起作用？

Question

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;
import javax.swing.event.*;


public class Main {
public static void main(String[]args){
    @SuppressWarnings("unused")
    Gui g = new Gui();
}
}
@SuppressWarnings("serial")
class Gui extends JFrame{
Dimension screen = Toolkit.getDefaultToolkit().getScreenSize();
double width = screen.getWidth();
double height = screen.getHeight();
JPanel canvas = new JPanel();
Point mloc = new Point();

JButton br = new JButton("Red");
JButton bb = new JButton("Blue");
JButton bg = new JButton("Green");
JButton wipe = new JButton("Wipe");

JLabel brushwidth = new JLabel("Width = ",JLabel.CENTER);

public JSlider s = new JSlider();

JButton image = new JButton("Image");

Point start = null;
Point current = null;
boolean entered = false;
Color c = Color.red;
public double bwidth = 3;

Gui(){
    super("PaintPot");
    setSize((int)width/4,(int)height/2);
    setVisible(true);
    setResizable(false);
    setLayout(null);
    setLocationRelativeTo(null);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    add(s);
    s.setSize(getWidth()-20, 20);
    s.setLocation(10, 450);
    s.setBackground(Color.gray);
    s.setForeground(Color.orange);
    s.setMajorTickSpacing(50);
    s.setMinorTickSpacing(1);
    s.setValue((int)bwidth);
    s.addChangeListener(new ChangeListener(){
        @Override
        public void stateChanged(ChangeEvent e) {
            bwidth = s.getValue();
            brushwidth.setText("Width = "+s.getValue());
        }
    });
    brushwidth.setText("Width = "+s.getValue());

    add(brushwidth);
    brushwidth.setSize(70,30);
    brushwidth.setLocation(90,410);
    brushwidth.setBackground(Color.gray);
    brushwidth.setForeground(Color.orange);
    brushwidth.setOpaque(true);
    brushwidth.setVisible(true);

    add(wipe);
    wipe.setSize(70,30);
    wipe.setLocation(10, 410);
    wipe.setBackground(Color.gray);
    wipe.setForeground(Color.orange);
    wipe.setVisible(true);

    add(br);
    br.setSize(60,30);
    br.setLocation(10, 10);
    br.setBackground(Color.red);
    br.setForeground(Color.white);
    br.addActionListener(new ActionListener(){
        @Override
        public void actionPerformed(ActionEvent arg0) {
            c = Color.red;
        }
    });
    br.setVisible(true);

    add(bb);
    bb.setSize(60,30);
    bb.setLocation(80, 10);
    bb.setBackground(Color.blue);
    bb.setForeground(Color.white);
    bb.addActionListener(new ActionListener(){
        @Override
        public void actionPerformed(ActionEvent e) {
            c = Color.blue;
        }
    });
    bb.setVisible(true);


    add(bg);
    bg.setSize(70,30);
    bg.setLocation(150, 10);
    bg.setBackground(Color.green);
    bg.setForeground(Color.white);
    bg.addActionListener(new ActionListener(){
        @Override
        public void actionPerformed(ActionEvent e){
            c = Color.green;
        }
    });
    bg.setVisible(true);

    add(image);
    image.setSize(70,30);
    image.setLocation(230, 10);
    image.setBackground(Color.gray);
    image.setForeground(Color.orange);
    wipe.addActionListener(new ActionListener(){
        @Override
        public void actionPerformed(ActionEvent e){ 

        }

    });
    image.setVisible(true);
    canvas(this,canvas);

}
public void canvas(JFrame f, JPanel p){
    p.setSize(425,350);
    p.setBorder(BorderFactory.createLineBorder(Color.black, 3));
    p.setLocation(10, 50);
    p.addMouseListener(new MouseListener(){
        @Override
        public void mouseClicked(MouseEvent arg0) {}
        @Override
        public void mouseEntered(MouseEvent arg0) {entered = true;}
        @Override
        public void mouseExited(MouseEvent arg0) {entered = false;}
        @Override
        public void mousePressed(MouseEvent arg0) {}
        @Override
        public void mouseReleased(MouseEvent arg0) {}
    });
    p.addMouseMotionListener(new MouseAdapter(){
        public void mouseDragged(MouseEvent e){
            mloc = e.getLocationOnScreen();
            if(entered = true){
                paintComponent(getGraphics());
            }
        }
    });


    f.add(p);
}
public void paintComponent(Graphics g){
    g.drawOval(mloc.x, mloc.y, (int)bwidth, (int)bwidth);
}
}

I'm trying to get this app to paint inside the JPanel but I can't seem to get it to work, I want it to draw a line whenever I move my mouse. It's just the public void paint bit I can't get to grips with nothing seem to work. Thanks

Answer 1

JFrame does not have a paintComponent() method.

Custom painting is done by overriding the paintComponent() method of a JPanel (or JComponent) and then you add the panel to the JFrame.

Read the section from the Swing tutorial on Custom Painting for more information and examples. You will also need to override the getPreferredSize() method.

Also, don't use a null layout. Swing was designed to be used with layout managers.

Answer 2

1. This if(entered = true){ is an assigment operator not a conditional. Instead you want if(entered == true){

2. paintComponent is meant to be overriden and not called explicitly. Don't explicitly call paintComponent when what you mean to do is call repaint()

3. JFrame has no paintComponent method, so you aren't actually overriding any paint functionality. For JFrame you should override paint , though I'd advise against it, and paint with JPanel or JComponent

4. In a paintComponent or paint method you should also be calling super.paintComponent or super.paint , respectively, as to not break the paint chain.